Insulating window structures of the double-pane type generally comprise a frame surrounding a pair of glass panes which are spaced apart by the frame structure so as to define a gas-filled space between them.
This structure includes an inner pane, an outer pane as well as the spacing frame and can also include a resilient means for mounting, supporting and engaging at least one of these panes to enable relative displacement of this pane with respect to the other pane and the frame to a limited degree.
As is the case with all resilient or spring structures, the resilient means has a spring characteristic, i.e. a restoring force which is a function of its displacement, the resilient characteristic being expressed generally in terms of a characteristic curve (hereinafter spring force or restoring force characteristic) plotted on a graph whose ordinate represents the restoring force, spring force or pressure, while the abscissa represents the displacement of the resilient means.
As applied to the resilient means which can be utilized to retain a glass pane in the frame, the spring characteristic can be a plot of pressure applied by the resilient means to the spring-supported pane (along the ordinate) versus displacement of the resilient means by the movement of the pane along the abscissa.
The force supplied to the movable pane or, when both panes are movable, to the movable panes, thus represents a function of the resistance generated by the or each resilient means upon the displacement of the panes. This relative displacement of the panes can be toward or away from one another and can result from pressure changes such as a change in ambient or atmospheric pressure, or the effect of a sound field upon the window structure.
Either the inner glass pane or the outer glass pane, or both can be formed, in turn, of a number of glass sheets in a spaced-apart relationship and with gas-filled or evacuated spaces between them. Although each pane will be referred to in the singular herein, it should be understood that either or both of them may be compound panes made up of two or more glass sheets in the manner described.
German patent document No. 25 26 438 describes a window structure in which the resilient means is formed from a profile rubber strip which has a spring and a working point, i.e. the point along this curve with which the strip supports the movable glass pane at rest, i.e. the standard pressure in the absence of the sound field.
This characteristic and the working point are such that even with slight displacements of the movable pane by the effects described above, relatively large restoring forces are applied to the edges of the movable glass pane, particularly along the edge zones engaged by the resilient means.
Of course, with the conventional system described it is possible to reduce the restoring force with small displacements but only at the expense of "softening" the mounting of the movable pane, i.e. failing to generate a sufficient restoring force in the case of relatively large displacement of the pane.
We have found that because of this defect in the conventional systems, a window structure constituted as described has relatively little acoustic damping effect, i.e. does not materially damp or attenuate sound energy impinging upon one of the panes so that a considerable part of this energy is transmitted through the window structure to the other pane and thence into the region in which this latter pane is exposed.
In practice it is found that the spring characteristic in the region of the working point is so steep (stiff spring characteristic) that it cannot contribute more than two decibels (dB) to the damping effect. In addition, the window structure is highly susceptible to alteration of the resilient structure because of aging and hence to changes in the acoustic damping and other properties of the window.